1. Field of the Invention
The invention relates to a speed change gear.
2. Discussion of Background
A speed change gear is used as a speed reducing gear or a speed increasing gear that changes the speed of rotation input from a motor, or the like. As such a speed change gear, there is a known speed reducing gear that uses a planet gear mechanism in order to obtain a high change gear ratio, which is, for example, described in Japanese Utility Model Application Publication No. 60-127150 (JP-U-60-127150). The speed reducing gear described in JP-U-60-127150 oscillatingly rotates a pair of planet gears in the same phase with the rotation of an input shaft and then outputs the differences in rotational speed between external gears of the respective planet gears and internal gears meshed respectively with the external gears as a speed reducing ratio. A speed reducing gear described in Japanese Patent Application Publication No. 2002-266955 (JP-A-2002-266955) oscillatingly rotates a planet gear with the rotation of an input shaft and then transmits and outputs only the axial rotation component of the planet gear. The thus configured speed reducing gears are able to obtain a high change gear ratio with the use of a single-stage planet gear mechanism.
However, as the change gear ratio of the speed change gear is increased, the transmission efficiency of driving force decreases. Therefore, the size of the speed change gear is increased in order to obtain a higher change gear ratio. In addition, in the case of a speed increasing gear that requires particularly high transmission efficiency, there is a possibility that self-lock may occur. Accordingly, a speed increasing ratio obtained from a single-stage gear is limited. Then, as described in Japanese Patent Application Publication No. 2010-019286 (JP-A-2010-019286), there is a known configuration where speed reducing gear units having the same configuration are arranged in tandem. The speed reducing gear described in JP-A-2010-019286 includes speed reducing gear units in multiple stages to increase the overall change gear ratio.
However, in the speed reducing gear described in JP-A-2010-019286, although driving forces transmitted by the respective speed reducing gear units coupled to each other are different from each other, the speed reducing gear units have the same configuration. Therefore, transmittable driving force of the speed reducing gear is limited as a whole. Furthermore, because a member, or the like, that couples the speed reducing gear units is required, the size of the speed reducing gear increases in the direction along the input/output axis.